A commonly encountered problem with protease-containing liquid detergents is the degradation of other enzymes in the composition by the proteolytic enzyme. The stability of the other enzyme upon storage and its performance can be impaired by the proteolytic enzyme.
Boric acid and boronic acids are known to reversibly inhibit proteolytic enzymes. A discussion of the inhibition of one serine protease, subtilisin, by boronic acid is provided in Philipp, M. and Bender, M. L., "Kinetics of Subtilisin and Thiolsubtilisin", Molecular & Cellular Biochemistry, vol. 51, pp. 5-32 (1983).
One class of boronic acid, peptide boronic acid, is discussed as an inhibitor of trypsin-like serine proteases, especially in pharmaceuticals, in European Patent Application 0 293 881, Kettner et al., published Dec. 7, 1988.
In liquid detergents built with citric acid or a water-soluble salt thereof, boric acid appears to complex with the citric acid/salt. It is believed that this adversely affects boric acid's function as a proteolytic enzyme inhibitor. The proteolytic enzyme then is free to degrade cellulase in the composition, rendering it less effective. The extent to which the citric acid/salt complexes with a boric acid derivative is believed to be a function of the type of derivative employed in the composition.
The effectiveness of boric acid as a proteolytic enzyme inhibitor can be increased by the addition of 1,2 propane diol. Without intending to be limited by theory, it is believed that a predominantly 1:1 molar boric/diol complex is formed which is capable of binding with the active site (serine) on the proteolytic enzyme and inhibiting it.
However, it has been found that the addition of boric acid to liquid detergents containing citric acid/salt and 1,2 propane diol does not significantly improve cellulase stability in the presence of protease unless the boric acid and diol are added to the composition prior to the citric acid. With this order of addition, the boric/diol mixture is an effective protease inhibitor even in the presence of citric acid or a salt thereof. This minimizes degradation of the cellulase by the proteolytic enzyme. Upon dilution in water, such as under typical wash conditions, the proteolytic enzyme is no longer inhibited and can function to remove protease-sensitive stains from fabrics. The importance of adding boric acid and diol to liquid detergent compositions containing proteolytic enzyme and cellulase prior to adding citric acid/salt is not disclosed by the art. It is particularly surprising that the citric acid/salt does not render the boric/diol complex ineffective for protease inhibition over time.
European Patent Application 0 381 262, Aronson et al, published Aug. 8, 1990, discloses mixtures of proteolytic and lipolytic enzymes in a liquid medium. The stability of lipolytic enzyme is said to be improved by the addition of a stabilizer system comprising boron compound and a polyol capable of reacting with it, whereby the polyol has a first binding constant of at least 500 l/mole and a second binding constant with the boron compound of at least 1000 l.sup.2 /mole.sup.2.
German Patent 3 918 761, Weiss et al, published Jun. 28, 1990, discloses a liquid enzyme concentrate which is said to be usable as a raw material solution for making liquid detergents and the like. The concentrate contains hydrolase, propylene glycol and boric acid or its soluble salt.
U.S. Pat. No. 4,900,475, Ramachandran et al, issued Feb. 13, 1990, discloses a stabilized enzyme-containing detergent containing surfactant, builder salt and an effective amount of enzyme or enzyme mixture containing protease and alpha-amylase enzymes. The composition also contains a stabilization system comprised of glycerine, a boron compound and a carboxylic compound with 2-8 carbon compounds.
U.S. Pat. No. 4,537,707, Severson, Jr., issued Aug. 27, 1985, describes heavy duty liquid detergents containing anionic surfactant, fatty acid, builder, proteolytic enzyme, boric acid, calcium ions and formate. The combination of boric acid and formate provides improved proteolytic enzyme stability in the compositions.
European Patent Application 0 080 223, Boskamp et al, published Jun. 1, 1983, describes aqueous enzymatic detergent compositions containing boric acid or an alkali metal borate with a polyfunctional amino compound or a polyol, together with a reducing alkali metal salt.
Similarly, in GB 2 079 305, Boskamp, published Jan. 20, 1982, it is disclosed that enhanced enzyme stability can be obtained in a built liquid detergent composition by inclusion of a mixture of boric acid and polyol in a weight ratio of more than 1:1, and a cross linked neutralized polyacrylate polymer.